Fresh water tank overflow?

[Heinz Wolff]

Both fresh and grey water tanks hold 42 litres. When filling the fresh water tank, what happens when you inadvertently try to put a 43rd litre in? I've examined the tanks under the kitchen counter and found no overflow pipe. There's a seal around the access hole's lid and on each of the 2 narrow pipes going through the top of the tank (one of which seems to be to allow air out as you fill the tank). The filling pipe is widest, attached by a jubilee type clip. This pipe itself would fill up with water when the tank is full, you'd notice water flowing out but how much do you trust those various seals? The weakest link is the grommet (not intended to form a seal) around the wire to the sensor that provides the level reading to the control panel).
I can only assume that person filling the tank is required to somehow watch the control panel and stop before the tank is full?

 

[Heinz Wolff]

Update - I filled the tank yesterday, giving it a dose of Puriclean and accidentally caused it to overflow. I'd already taken the shelf and cushion off and I can confirm that your first sign is water coming out of the filler. All the seals worked, no water lost inside the van BUT there's no way it would stay dry if the van was moved.
Easy solution was to drain some off using the valve under the fridge.
All I've got to do know is work out how to get the water pump to work. The power supply is on (tried battery and mains) and I can hear the click as I open the tap... nothing.